The development in recent years of methods for the chemical analysis of the fatty constituents of blood has made possible the study of the partition of cholesterol and lecithin between the blood plasma and cells, a matter of great interest in view of the reported antagonism existing between these substances. This article is part of a group of studies on the concentration and distribution of these lipoids in normal and pathologic blood, and the relation of these factors to certain physicochemical properties of the red blood cells possibly dependent on them. It is also planned to study from this standpoint the balance between free and combined cholesterol. The investigation herein reported is concerned only with the lipoid partition in the blood of normal subjects and of patients suffering from a variety of conditions in which abnormalities in blood lipoids were expected. It has been reported by Bloor,1Chauffard, Laroche and